The coagulation stage of milk is the most important and fundamental treatment step in, for example, the production of cheese. The state of coagulation essentially governs the quality of the resulting final product. Given these circumstances, the determination of the state of coagulation of milk has conventionally been carried out subjectively on the basis of the experience of skilled technicians. On the other hand, a number of measurement methods have also been invented to measure the coagulation of milk by means of instruments. However these methods are accompanied by such drawbacks as the milk coagulum has to be squeezed or pressed to cause it to change shape, and in some instances, cause whey to be released. As a result, measurements are made on the coagulated milk in a state different from its normal state. Use of such measurement methods are thus limited to research purposes, and they are impractical for actual application in the production of, for example, cheese.